Only update class docstring if ipython is available

[jlstevens]

Currently param updates class docstrings to help with tab completion. I think this behavior is useful (and can be disabled by setting the docstring_signature variable to False but it might be worth considering two tweaks to this behavior:

1. Only doing this for ParameterizedFunction (might be tricky, currently the logic is on ParameterizedMetaclass)
2. Only doing this when in an ipython context. This may also be hacky as it might mean trying to detect get_ipython in the namespace (or similar).

I just want to consider these options as I would prefer to mutate docstrings only when tab-completion is actually relevant.

[maximlt]

@jlstevens was this resolved by #774? If so, I'll let you have the pleasure to close this old issue.